A trap play is a play where the defensive player is allowed to cross the line of scrimmage and then blocked in the backfield by a pulling lineman from somewhere else on the line (usually a guard).

It's called a trap because the defender is TRICKED into thinking he's in the backfield alone because the o-lineman across from him does not block him. The defender is then blocked by the pulling lineman and ideally, the runnuing back runs through the hole where the 'trapped' d-lineman used to be.